Why doesn't struct exist after declaration inside of function?

Question

I've tried a lot of simple examples of this and haven't gotten any to work.

My goal is to have a function which declares a struct internally, sets the values of the struct, and then returns the struct.

struct getData(void){
    typedef struct{
        int count1;
        int count2;
    } MyStruct;

    MyStruct myData;

    myData.count1 = 5;
    myData.count2 = 6;

    return myData;
};

int main(void) {
    struct myData = getData()

    printf("count1: %i", myData.count1);
    printf("count2: %i", myData.count2);
}

Every example I've found does something similar to this, but for some reason it's not finding my struct called MyStruct. Exact error is:

error: expected identifier or ‘(’ before ‘void’
 struct getData(void){
                ^~~~

The error I keep getting makes me think it doesn't like the struct inside the function.

The keyword struct needs to be followed by either the name of a previously declared structure type, or the specification of the sdtructure. You can't just declare a variable or function as returning an unknown type of structure. — Barmar
– Barmar, Commented Oct 2, 2019 at 20:59
If you want to use a struct outside the function its definition must be visible there. Therefore you can't define it inside the function. You can define a variable of this struct type inside the function and return it, though. — the busybee
– the busybee, Commented Oct 2, 2019 at 21:00
The curly brackets represent scope of visibility of symbols. So you can't expect something declared in an inner scope (the function) to be visible on the outside. — Ivaylo Valchev
– Ivaylo Valchev, Commented Oct 2, 2019 at 21:03
With the struct type defined local within getData what form of magic do you expect to use outside the function to access the members set within? Amore fundamental question: What are you really trying to accomplish? I ask because this seems very much an XY problem, where you have a problem, decided firmly on a solution, tried the solution, and upon failing, decided the problem is within your solution; not the original problem itself. So.. what are you really trying to accomplish here? — WhozCraig
– WhozCraig, Commented Oct 2, 2019 at 21:19

This isn't my real name · Accepted Answer · 2019-10-02 21:24:10Z

1

Your problem seems to be a confusion regarding the usage of the struct keyword. You don't do struct myData to declare a variable named myData that is of struct type, because there isn't really a struct type. What you do is struct myData <SOMETHING> to define <SOMETHING> as being a new data type named struct myData. You can then say struct myData dat;, thereby declaring that dat is a variable of type struct myData.

You're also demonstrating the same confusion at the top, where you have struct getData(void)... you're attempting to declare getData as a function returning a struct, but you'd really have to do something like struct myData getData(void) to declare a function returning type struct myData.

answered Oct 2, 2019 at 21:24

This isn't my real name

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Comments

neutrino_logic · Accepted Answer · 2019-10-02 21:29:29Z

0

I think the more standard way to write what you're after is this, using an independent definition of the struct, and then using a pointer for the function call. As written your code seems to have some scope issues (declaring a struct typedef inside a function, hmmm...) [Incorrect->] plus I don't believe structs can be passed around that way in C, I'm pretty sure you have to use pointers. [Edit oops this is incorrect! See comments]

#include<stdio.h>

typedef struct {
    int count1;
    int count2;
    } MyStruct;

void getData (MyStruct* myDataPtr) {
    myDataPtr->count1 = 5;    //note pointer access notation ->
    myDataPtr->count2 = 6;
};

int main(void) {
    MyStruct myData;
    MyStruct* sPtr = &myData;  //create pointer to struct, assign to myDaya
    getData(sPtr);            //pass pointer to function
    printf("count1: %i \n", myData.count1);
    printf("count2: %i \n", myData.count2);
}

Outputs:

count1: 5
count2: 6

edited Oct 2, 2019 at 21:29

answered Oct 2, 2019 at 21:18

neutrino_logic

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4 Comments

WhozCraig Over a year ago

"I don't believe structs can be passed around that way in C" - you're wrong. They can be passed around by value without issue. MyStruct getData() is completely viable, and in fact one common technique for returning an array by value from a function (i.e. bury it in a structure type).

Christian Gibbons Over a year ago

structs can be passed just fine. Only issue is that if it's really big then that there could be a lot of data being copied.

neutrino_logic Over a year ago

Thanks, I had no idea for some reason... can you do that in C++ too? And you can bury an array in a struct and pass it about, too... I wonder if the scope issue is a problem then?

dmckee --- ex-moderator kitten Over a year ago

You can also bury an array in a struct and assign one of those things to another to get around the lack of array assignment. It fills your code up of long-winded and ugly constructs, but every once in a while it's worth it. (And given the extreamely long-winded nature of some of the c++ code I'm producing thiese days aryStruct.ary[i] doesn't seem so bad anymore.)

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